As integrated circuit manufacturing technology has advanced, it has become possible to decrease the size of electronic devices. Great efforts have successfully been directed toward the miniaturization of many different types of electronic devices (e.g., telecommunications equipment). One of the limitations on the degree of miniaturization possible is the construction and design of the control surface (user interface). Suitable interface devices for enabling a user to interact with functionally highly complex, miniaturized electronic circuitry have become necessary. A suitable control interface must necessarily have a high control per unit area density if the overall size of the electronic device is to be minimized. The interface should suitably be capable of providing the user with appropriate information about operating parameters of the device and permitting the user to program a wide variety of control data into the electronic device.
It is often desirable that such a control interface be durable, shock-resistant, weather-resistant and highly reliable under a variety of adverse conditions in order to withstand rough treatment.
Matrix alpha-numeric keyboards are especially suited for high density control surfaces because they provide a very high control density per unit area and are useful for programming a variety of different types of information (such as numerals, strings of numerals, alphabetic symbols, strings of alphabetic symbols, and control symbols). The keys of such alpha-numeric keyboards typically are opaque. Each key typically bears one or more symbols designating its function or functions.
One of the disadvantages of alpha-numeric keyboards is that they may be difficult to operate under low ambient light conditions. At low light levels, the user may not be easily able to distinguish between the various keys, or to read the symbols imprinted on the keys. Western Electric Company, among others, has developed a TOUCH-TONE keypad providing backlighting of the keys for ease of operation under low ambient light conditions. The Western Electric Co. TRIMLINE.RTM. model telephone set is equipped with such a keypad. Each of the keys of the keypad comprises translucent material (such as plastic). A lighting source is provided behind the keypad to backlight the keys, providing high contrast between the key and opaque symbols imprinted on each of the keys. Keys may be distinguished from one another, and the function of each key may be easily ascertained.
Unfortunately, while conventional TOUCH-TONE keypads provide versatility, high reliability and durability, they typically require a substantial mounting surface area. Likewise, the backlighting technique typically employed for such keypads limits the degree of miniaturization of the control surface (because of the extra volume behind the keypad occupied by the components providing backlighting). For this reason, past keypads have been unsuitable for extremely miniaturized electronic devices encased in housings having very small surface areas.
Another limitation on the miniaturization of electronic devices in the past has been the power and mounting area requirements of versatile display devices. Sophisticated miniaturized electronic devices often may generate information (such as, for example, user-entered data, various operating parameters, status of operation, etc.) the display of which may be useful to the user. Much of this information may not be indicated by simple on/off indicators (historically the type of visual indicators requiring the smallest mounting area). Light Emitting Diode seven-segment digital displays have been developed that required only a small mounting area, and yet can display a wide variety of alpha-numeric information. Such LED displays, however, consume relatively large amounts of power (typically 10-40 mW/digit); thus, the decrease in the surface area of the control surface made possible by the use of an LED digital display device is often counteracted by the added volume necessary to accommodate the associated power supply requirements of the display device.
There has been an increasing growth in the use of liquid crystal displays, which provide alpha-numeric readouts with very low power consumption (on the order of 150 .mu.W for a seven segment, 10-mm digit). As is well known, a liquid crystal display provides a visual indication by selectively changing its light transmissivity (rather than by actually producing light). A reflecting surface provided behind a liquid crystal display typically reflects enough ambient light to provide illumination of the displayed indication. Under low ambient light conditions, however, a source of light must be provided in order to make the indication visible.
A liquid crystal display is typically mounted on a substantially flat control surface. Due to the properties of typical liquid crystal displays, the angle of view is often limited to only 45.degree. from normal, requiring that illuminating light strike the display at an angle close to normal to the face of the display. For this reason, liquid crystal displays are typically backlighted if they are to be used under low ambient light conditions. Often, backlighting is accomplished by positioning a half-silvered mirror behind the display. The mirror acts as a reflector for ambient light and as a partially transmissive medium for a light source positioned behind the mirror. This technique produces a contrast in the indication which is not as good as can be obtained from either a purely transmissive or a reflective system, although the comprise is adequate for some purposes. See Sherr, Sol, Electronics Displays at page 498 (John Wiley & Sons, Inc., 1979).
A further disadvantage of this backlighting technique is that space must be provided behind the liquid crystal display for a light source. A separate light source is typically used solely for backlighting the liquid crystal display. Where space is at a premium, it is often undesirable and difficult to position a light source behind the liquid crystal display. A light source positioned behind the display may interfere with the electrical connections between the display and its driving circuit, and the light source may be difficult to replace when it fails. The present invention provides a backlighted display and an illuminated keypad requiring only very small surface mounting area and having none of the above disadvantages.